1. Field of the Invention
The present invention concerns an optical article such as a lens or a mirror, an exposure apparatus or optical system such as a stepper having it, and a process for producing the optical article. More particularly, the invention relates to an optical thin film for an optical article suitably applicable to an exposure apparatus or optical system using an excimer laser, and a process for producing it.
2. Related Background Art
Optical articles, including lenses, mirrors, and optical filters, are used, for example, in optical apparatus such as cameras, telescopes, and microscopes. These optical articles have an antireflection film or a reflection-enhanced film on the surface thereof for prevention of reflection or for enhancement of reflection.
An exposure apparatus is a kind of an optical apparatus equipped with such an optical article. The exposure apparatus is used in the fabrication steps of semiconductor integrated circuits or photomasks for fabricating them. A typical example of an exposure apparatus in this field is an exposure apparatus called a stepper.
An illumination light source in such an exposure apparatus, conventionally used, was a super-high pressure mercury lamp, a xenon-mercury arc lamp, or the like for emitting the g-line (435.8 nm), the h-line (404.7 nm), and the i-line (365 nm). However, attempt has recently been made to use lasers emitting far-ultraviolet rays (200 to 260 nm) or emitting a beam of high power and narrow spectral width in order to realize high exposure processing capability per unit time (throughput) and uniform illumination characteristics on an exposed body such as a wafer. Among others, an excimer laser is one of desirable light sources, because it emits light in a very narrow spectral width and at high output power.
One of known optical articles for an excimer laser is the one having an antireflection film deposited by vacuum vapor deposition as disclosed in Japanese Laid-open Patent Application No. 63-113501 or Japanese Laid-open Patent Application No. 63-113502.
However, for example, if a lens having sufficient optical characteristics in the optical system for visible light was used in an optical system for an excimer laser, it was sometimes difficult to maintain optical characteristic sufficient to be applied to practical use. Specifically speaking, transmission characteristics, particularly, of the optical thin film provided over the surface of a lens or mirror were not sufficient to make the best use of the advantages of the excimer laser, and there was a problem in the durability thereof.
In addition, since the optical articles for an exposure apparatus were required to have a surface formed at very high accuracy, strict control of temperature conditions was necessary in forming the optical thin film. Thus, film forming techniques generally regarded as being preferable were not able to be used as they were in fabricating such optical articles.
As explained above, in order to fabricate an optical article that is durable against the use of an excimer laser, the optical article must be designed based on a novel idea and a novel approach.
The present invention has been accomplished in view of the technical problems discussed above, and an object of the invention is to provide an optical article having optical characteristics fully durable against such hard operation conditions as in applications with an excimer laser.
Another object of the present invention is to provide an optical article with an optical thin film having little unnecessary light absorption and having uniform physical properties over a large area.
Still another object of the present invention is to provide an optical article with an optical thin film which can be formed at low temperature and which is free from the negative effect of stress such as film separation.
A further object of the present invention is to provide an optical system and an exposure apparatus having the above optical article excellent in characteristics.
Of course, the one durable to the excimer laser must exhibit adequately good characteristics also in optical systems using the other light.
The optical article of the present invention is an optical article in which an optical thin film is laminated on a surface of a substrate, wherein the optical thin film comprises atoms of at least one selected from the group consisting of krypton, xenon, and radon.
The optical article of the present invention is an optical article in which an antireflection film comprising a first optically transparent thin layer and a second optically transparent thin layer having a higher refractive index than that of the first optically transparent thin layer are formed on a surface of an optically transparent substrate, wherein at least either one of the first and second optically transparent thin layers comprises atoms of at least one selected from the group consisting of krypton, xenon, and radon.
Further, the optical article of the present invention is an optical article in which a reflection-enhanced film comprising a first optically transparent thin layer and a second optically transparent thin layer having a lower refractive index than that of the first optically transparent thin layer are formed on a surface of a substrate, wherein at least either one of the first and second optically transparent thin layers comprises atoms of at least one selected from the group consisting of krypton, xenon, and radon.
The optical article of the present invention may be adaptive to the arrangement further having a third optically transparent thin layer having a different refractive index from those of the first and second optically transparent thin layers. The first optically transparent thin layer is preferably silicon oxide, and the second optically transparent thin layer is preferably aluminum oxide or tantalum oxide. The content of the atoms of the selected element in the optically transparent thin layer is not more than 5 atomic %, and the contents of the element in the first and second optically transparent thin layers are different from each other. The article is characterized in that the substrate comprises silica or fluorite. Further, the optical article of the present invention is characterized in that it selectively transmits or reflects the excimer laser light.
The exposure apparatus of the present invention is an exposure apparatus having an illumination light source and a stage for an exposed object to be mounted thereon in which an illumination optical system and/or a projection optical system comprises a plurality of optical articles in each of which a first optically transparent thin layer and a second optically transparent thin layer having a higher refractive index than that of the first optically transparent thin layer are laminated on a surface of a substrate, wherein at least either one of the first and second optically transparent thin layers comprises atoms of at least one selected from the group consisting of krypton, xenon, and radon. The apparatus is characterized in that the illumination light source is an excimer laser light source. Further, the apparatus is characterized in that the optical article has a characteristic to selectively transmit a plurality of laser light beams of different wavelengths from each other.
The process for producing the optical article of the present invention is a process for producing an optical article in which a first optically transparent thin layer and a second optically transparent thin layer having a higher refractive index than that of the first optically transparent thin layer are laminated on a surface of a substrate, wherein at least either one of the first and second optically transparent thin layers is deposited by sputtering using a sputtering gas comprising atoms of at least one selected from the group consisting of krypton, xenon, and radon. The process is characterized in that a gas for oxidation is used as a reaction gas in addition to the sputtering gas. Further, it is preferred that the substrate be held at 100xc2x0 C. or less in the sputtering and that before the sputtering at least a film-forming surface of the substrate be held at 100xc2x0 C. or less under a nitrogen gas atmosphere.
The present inventors have accomplished the present invention by finding out that, as compared with films not containing krypton, xenon, or radon, obtained by vacuum vapor deposition or sputtering with Ar gas, films containing atoms of at least one of these three elements can maintain good transmittances for a longer period.
The invention can provide an optical article having an optical thin film which is less in unnecessary light absorption, uniform over a large area, free of film separation, and excellent in durability.
The invention can provide an optical article having excellent transmission characteristics of light used, because the optical thin film is used as an antireflection film.
The invention can provide an optical article having excellent reflection characteristics of light used, because the optical thin film is used as a reflection-enhanced film.
The invention can achieve an optical article having better transmittance stability, and particularly having excellent applicability to the excimer laser optical system.
The invention can achieve an optical article having better transmittance stability, and particularly having excellent applicability to the i-line optical system.
The invention makes a non-monocrystalline film finer and reduces degradation of transmission characteristics more by the feature that the content of xenon, krypton, or radon is not more than 5 atomic %.
The invention improves adhesion and enhances the antireflection or reflection-enhancing effect.
The invention can provide an excellent excimer laser optical system.
The invention can provide an excellent excimer laser reflection optical system.
The invention hardly causes an error in exposure parameters, and it enables good photolithography and enhances reliability.
The invention enables high-definition and high-accuracy photolithography.
The invention enables automatic alignment with high accuracy and high reliability.
The invention enables formation of uniform plasma, whereby an optical article with uniform characteristics and in a large area can readily be produced at the low temperature of not more than 150xc2x0 C., preferably not more than 100xc2x0 C.
The invention has the feature that the gas having the oxidation effect is introduced as a reaction gas, which provides the ability to obtain an oxide film with more excellent characteristics in a stoichiometric composition or in a composition very approximate thereto, even though it is a non-monocrystalline film.
The invention can suppress a change of the surface shape of the optical article. Further, gas emission becomes less during formation of a film, which can suppress occurrence of film separation or unnecessary products.
The invention can suppress gas emission during formation of a film by exposing the substrate to the nitrogen gas atmosphere before sputtering.
The invention can provide an optical system having excellent transmission characteristics of light used.
The invention can provide an optical system excellent in reflection characteristics of light used.